1. Field of the Invention
The present invention relates to an image forming method and apparatus for producing a fluid having a predetermined density and/or a predetermined color by changing a proportion or mixing ratio of a plurality of inks based on an image signal and leading the thus obtained fluid to an image receiving medium to form an image. Further, the present invention relates to a recording head for use in this image forming apparatus.
2. Description of the Prior Art
U.S. Pat. No. 4,109,282 (which will be referred to as a prior art reference 1, hereinafter) discloses a printer having a structure such that a valve called a flap valve is provided in a flow channel for leading two types of liquid, i.e., clear ink and black ink onto a substrate for forming an image. The flow channel for each ink is opened/closed by displacing this valve so that the two types of liquid are mixed in a desired density to be transferred onto the substrate. This enables printout of an image having the gray scale information which is the same as that of the image information displayed on a TV screen. In this reference is disclosed that a voltage is applied between the flap valve and an electrode provided on a surface opposed to the flap valve and the valve itself is mechanically deformed by the electrostatic attracting force to cause displacement of the valve. Further, the ink is absorbed in paper by a capillary phenomenon between fibers of the print paper.
U.S. Pat. No. 4,614,953 (which will be referred to as a prior art reference 2, hereinafter) discloses a printer head apparatus by which only a desired amount of multiple types of ink having different colors and solvent is led to a third chamber to be mixed therein. In this reference is disclosed that a chamber and a diaphragm-type piezoelectric effect device attached to this chamber are used as means for check-weighing a desired amount of ink and a pressure pulse obtained by driving this piezoelectric device is utilized.
Unexamined Japanese Patent Publication (KOKAI) No. 201024/1993 (which will be referred to as a prior art reference 3, hereinafter) discloses an ink jet print head including: a liquid chamber in which a carrier liquid is filled; ink jet driving means provided in the liquid chamber; a nozzle communicating with the liquid chamber; and a mixing portion for mixing ink to the carrier liquid in this nozzle. In this reference is also disclosed that adjusting means for adjusting an amount of mixture of ink to a desired value is provided.
Similarly, Unexamined Japanese Patent Publication (KOKAI) No. 125259/1995 (which will be referred to as a prior art reference 4, hereinafter) discloses an ink jet recording head including: first and second supplying means for supplying inks having first and second densities, respectively; and controlling means which controls an amount of supply of the second ink by the second supplying means so that a desired ink density can be obtained.
In this reference 4, employment of a micro-pump which has an exclusive heating device and is driven by its heat energy is disclosed. As this micro-pump, there is disclosed an example such that the heat energy is generated by the heating device and a pressure obtained by the nucleate boiling caused due to the heat energy is used to drive, e.g., a piston-type valve or a cantilever-like valve. Further, this reference 4 describes that an inflow of ink can be effectively controlled in an area where the inflow is particularly small by adopting an actuator consisting of shape memory alloy to this valve.
Unexamined Japanese Patent Publication (KAKAI) No. 207664/1991 (which will be referred to as a prior art reference 5, hereinafter) discloses a structure which is similar to that in the prior art reference 2 but does not use a third chamber for mixing a plurality of types of ink.
Unexamined Japanese Patent Publication (KOKAI) No. 156131/1997 (which will be referred to as a prior art reference 6, hereinafter) discloses an ink jet printer comprising a plurality of printer heads for forming an image having multiple colors based on image data. Ink and diluent are mixed to obtain diluted ink which is jetted from a nozzle so that a recording image is formed on a recording medium. The ink jet printer ejects the diluent from at least one printer head out of the multiple printer heads when all-white image data, that is, data representing that amount of mixture of ink is too small to realize a clear printing density, is input. As a result, a rapid change in tone (a tone jump) is prevented and the additional consumption of the diluent is suppressed to improve drying characteristics.
Unexamined Japanese Patent Publication (KOKAI) No. 264372/1998 (which will be referred to as a prior art reference 7, hereinafter) discloses employment of a plurality of line heads in which ink ejection nozzles are linearly aligned. In this example, when the respective line heads are biased and arranged in a direction for feeding print paper and positions of nozzles in the respective line heads are biased relatively to a direction of the width of the print paper, the pixel density can be enhanced. Further, ink having a single color is ejected from each nozzle, and ink droplets having different colors are combined by ejecting ink having different colors in accordance with the line heads, thereby representing predetermined colors on the print paper.
In the respective prior arts disclosed in the prior art references 1 to 6, the different types of ink are mixed in advance to be then ejected, and an amount of supply of at least one type of ink among the multiple types of ink to be mixed is controlled. Therefore, a quantity of flow of ink having a desired density after mixed, i.e., a volume flow rate per unit time varies in accordance with a change in density or color. It has been revealed that, when the volume flow rate (which is also referred to as a flow rate hereinafter) per unit time of the ink fluid after mixing fluctuates in accordance with a change in ratio of mixture due to density or color in this manner, the quality of a finally-formed image is prominently deteriorated.
That is, in the image forming technique adopting the conventional ink jet mode described above, a volume of droplets formed by one ejecting operation (the ejection volume) is substantially constant, whereas a liquid flow rate of the mixed ink which is newly sequentially supplied to an ejection port (a jet generating portion) fluctuates. For example, when a supplied flow rate of the mixed ink is large, the supplied amount of the ink exceeds a quantity of droplets which can be ejected by one ejection operation, and the liquid remaining in the ejection port is mixed in the droplets for the next pixel. Further, when a supplied flow rate of the mixed ink is small, a part of the droplets for the next pixel is disadvantageously fetched. This adversely affects the image quality.
The applicants has been examining a mode for continuously transporting the ink liquid to an image receiving medium as a continuous flow without making droplets of the ink liquid (which will be referred to as a continuous coating mode hereinafter) in place of the ink jet mode. It has revealed that a fluctuation of an amount of supply of the mixed ink results in various problems as described above in this mode too. For example, when an amount of supply of the mixed ink liquid changes, a flow of the liquid may be disordered.
In the continuous coating mode, it is desirable that this liquid is transported to an image receiving medium as a steady flow. If there is an occurrence of a disorder or a whirlpool in this flow, the image quality is deteriorated. In addition, a fluctuation of an amount of supply of the liquid leads to coating layers having different thicknesses formed on the image receiving medium, but it is very difficult to stably form the coating layers having different thicknesses depending on the structure of a liquid ejection port. Even if formation of such coating layers is possible, irregularity is generated on the surface of the recorded image, thus deteriorating the image quality.
Since the ink having a single color is ejected from one nozzle in the prior art disclosed in the prior art reference 7, one pixel is formed by multiple (three, four or more colors) ink droplets. Therefore, the pixel density is hard to be enhanced, and improvement of the image quality is also restricted.
The present invention has been accomplished under the circumstances as aforementioned, and a first object thereof is to provide an image forming method for producing ink liquid having a desired density and/or color by mixing or combining a plurality of types of ink having different densities and/or colors and transporting this ink liquid to an image receiving medium to form an image, thereby improving the image quality.
Further, it is a second object of the present invention to provide an image forming apparatus which is directly used for implementing this method. Moreover, it is a third object of the present invention to provide a recording head for use in manufacturing of this image forming apparatus.
According to the present invention, the first object can be attained by an image forming method for ejecting a plurality of types of ink from an ink ejection port while changing a mixture proportion of a plurality of types of said ink based on an image signal and transporting a plurality of types of said ink to an image receiving medium which is displaced relatively to said ink ejection port to form an image; wherein a quantity of flow of the respective ink to said ink ejection port is controlled in such a manner that a total ejection volumetric flow rate of a plurality of types of said ink becomes always constant.
Print paper may be used as the image receiving medium, and an image can be directly formed on this print paper. However, it is possible to adopt a mode such that a drum-like or belt-like intermediate image receiving medium is provided between the ejection port and the image receiving medium such as a recording sheet and the ink liquid supplied from the ejection port is loaded on the intermediate image receiving medium, so that the ink liquid is then transferred to the image receiving medium. Preferably, the ink ejection ports may be separately provided in accordance with pixels aligned in a direction of the width of the image receiving medium (a direction orthogonal to the moving direction). The ink ejection ports may be formed into a slot-shaped opening which is elongated in a direction of the width of the image receiving medium when changing the density and/or the color only in the moving direction of the image receiving medium.
When it is determined that at least one type of ink is image non-forming ink, i.e., ink which is or becomes transparent and colorless after dried out (which will be referred to as image non-forming ink or clear ink hereinafter), the density can be controlled by changing a proportion or mixing ratio of the image non-forming ink in the ink liquid. It is preferable to add the image non-forming ink to the ink liquid any time so that the amount of supply of the image non-forming ink not become zero. In such a case, when a decoloration preventing agent such as antioxidant, ultraviolet ray absorber or any other component is included in the image non-forming ink in advance, a color degradation preventing property and others can be imparted to an image.
Transporting a plurality of types of ink with respect to one pixel from the same common ink ejection port to the image receiving medium can eliminate or minimize shift of the density or color of an image. However, a plurality of ink ejection ports may be separately formed in contiguity with each other with respect to one pixel. In other words, the respective types of ink may be confluent and mixed or combined in the vicinity of each ejection port. In addition, an image whose density and/or color changes in both the moving direction and the width direction of the image receiving medium can be formed by controlling a flow rate of a plurality of types of ink in accordance with respective pixels.
A plurality of types of ink ejected from the ink ejection port may be jetted on the image receiving medium as droplets by the ink jet mode. It is also possible transport a plurality of types of the ink to the image receiving medium as a continuous flow in place of the droplets (the continuous coating mode). In case of this continuous coating mode, a flow of the liquid can be ejected or extruded as a continuous flow and transported to the image receiving medium through a slot opening connecting the ink ejection ports provided for the respective pixels in the width direction.
A flow rate of a plurality of types of ink (an ink flow rate) can be controlled by the various methods. For example, an ink supply pressure with respect to each ink channel can be maintained constant while a cross sectional area of each ink flow channel can be changed by a piezoelectric device. In this case, a diaphragm valve facing to the flow channel is opened/closed by the piezoelectric device. The piezoelectric device can be driven by a mechanical natural frequency (a resonance frequency) of the device itself, and the time period for driving the device is changed by varying a pulse number of this frequency in order to control the flow rate. It is also possible to continuously control a quantity of distortion (an opening of the diaphragm valve) of the piezoelectric device by an analog signal and, in this case, the flow rate is controlled by a voltage of the analog signal.
When controlling all flow rates of a plurality of types of the ink by using the piezoelectric devices, a sum of cross sectional areas of the ink channels controlled by these piezoelectric devices is adjusted to be always constant. For example, a sum of the pulse number for the time period for driving each piezoelectric device is controlled to be constant or a total voltage of the analog signals is adjusted to be constant.
A flow rate supplied to each ink channel may be controlled by changing a discharged quantity of an ink feed pump. For example, the ink feed pump is driven by a pulse motor (a stepping motor), and the ink flow rate can be controlled by the driving pulse number of this pulse motor. The ink feed pump includes: at least one check valve provided to the ink channel; a cavity provided in the vicinity of this check valve; and a movable member for changing a volumetric capacity of the cavity, so that the pump discharges the ink by changing a volumetric capacity of the cavity. Such pump can be used as an ink feed pump.
The check valve used in the ink feed pump may be constituted by a geometrical form by which a resistance relative to the ink flow direction becomes small and that relative to the reverse direction becomes large. Such a check valve has no movable portion and can be produced by utilizing a method for manufacturing an integrated circuit or a printed wiring board or that for manufacturing a micro-machine. The ink feed pump may be driven by the pulse motor.
When the ink feed pump driven by the pulse motor is provided to each of a plurality of ink channels, a total flow rate of the ink liquid can be controlled to be constant by always maintaining a total driving pulse number of the pulse motor for driving each ink feed pump constant. Incidentally, the ink feed pump used in this example may preferably be of a volumetric capacity type by which an amount of ejection is proportionate to a quantity of rotation of the motor and, for example, a pump for squeezing a flexible tube appressed against the inner surface of a circular case from the inner peripheral side by an eccentric in a defined direction, a vane pump, a gear pump and others are suitable.
The ink feed pump provided to each ink channel may be formed by the piezoelectric device and the check valve. In this case, the piezoelectric device is a diaphragm valve driven by a mechanical resonance frequency inherent to the device. By controlling each piezoelectric device in such a manner that a sum of pulse number (pulse number in a defined period of time or a unit time) of the driving frequency of each piezoelectric device is always constant, a total ejected volume flow rate of the ink can be maintained constant.
According to the present invention, the second object can be attained by an image forming apparatus for ejecting a plurality of types of ink from an ink ejection port while changing a mixture proportion of a plurality of types of said ink based on an image signal and transporting a plurality of types of said ink to an image receiving medium which is displaced relatively to said ink ejection port to form an image, said image forming apparatus comprising:
ink flow controlling means for independently controlling ink flow rates of a plurality of types of said ink;
a processor for determining a mixture proportion of respective types of said ink based on the image signal and for calculating an ink flow rate of each ink by using the determined mixture proportion in such a manner that a sum of ink flow rates of respective types of said ink becomes a fixed ejection volume flow rate; and
a driver for driving said ink flow controlling means based on a result of calculation by said processor.
In order to control the ink flow rate, a diaphragm-type flow control valve driven by a piezoelectric device may be provided to the ink channel, for example. In place of the diaphragm valve driven by the piezoelectric device, a diaphragm valve driven by the heat-pressure effect or a counterpart driven the electrostatic attraction force or the electrostatic repulsive force may be used. In such a case, it is needless to say that the ink supply pressure with respect to the ink channel is always maintained constant. Additionally, a discharge quantity of the ink feed pump for supplying ink to the ink channel can be controlled without using the flow control valve. Preferably, such pump is of a volumetric capacity type which is driven by the pulse motor.
The ink flow rate controlling means may comprises: a check valve provided to the ink channel; a cavity provided in the vicinity of the check valve; and a movable member for changing a capacity of the cavity and have a structure for discharging the ink by varying a capacity of the cavity. In this example, the check valve may have a geometrical form such that an ink flowage resistance with respect to a flow direction of the ink becomes small while the same with respect to the reverse direction becomes large. The movable member can be constituted by a diaphragm driven by the piezoelectric device (or formed by the piezoelectric device itself). The movable member can be made up of a diaphragm driven using the heat-pressure effect, the electrostatic attraction force or electrostatic repulsive force, the magnetic distortion effect, the interfacial tension effect of fluid which is different from the ink, and others or a diaphragm driven by air bubbles generated by the electrolytic process of fluid which is different from the ink.
The ink ejection ports are arranged in accordance with pixels aligned in a direction of the width of the image receiving medium and they are independently opposed to the image receiving medium. In this case, the ink droplets can be transported by the ink jet mode. Additionally, in this case, the ink may be applied by the continuous coating mode in place of the ink jet mode. When using the continuous coating mode, the fluid ejected or extruded from each ink ejection port can be led to the image receiving medium through a slot opening which is elongated in a direction of the width of the image receiving medium. A flow of the ink liquid can be further stabilized as a steady flow to be lead to the image receiving medium by using the slot opening in this manner.
In the continuous coating mode, the liquid ejected from the ink ejection port may be transported to an intermediate image receiving medium such as a transfer drum, and the ink liquid can be transferred from this intermediate image receiving medium onto a final image receiving medium such as recording or print paper. As described above, the ink liquid ejected from the ink ejection port can be smoothly transferred by using the intermediate image receiving medium, and the deteriorated image quality due to the uneven quality of the image receiving medium such as print paper can be prevented from being generated.
According to the present invention, the third object can be attained by a recording head for use in the above-mentioned image forming apparatus, wherein ink ejection ports are arranged on a straight line which is orthogonal or substantially orthogonal to a relative displacement direction of an image receiving medium.
When the adjacent ink ejection ports are distributed to multiple parallel straight lines which are orthogonal or substantially orthogonal to the relative displacement direction of the image receiving medium, the pixel density can be enhanced.
In the present method and apparatus, a total flow rate of multiple types of ink (the volume flow rate per unit time) ejected from one ink ejection port is always maintained constant. Accordingly, conditions for transporting the mixed ink liquid to image receiving medium are satisfied, thereby performing the smooth transportation. For example, when adopted to the ink jet mode, a flow rate of ink newly supplied to the ink ejection port can be always matched with a volume of the jetted droplets. Therefore, an image having the high image quality can be stably formed without affecting the droplets with respect to the adjacent pixels.
Further, when the present apparatus is adopted to the continuous coating mode, the flow of the ink liquid from the ink ejection port or the slot opening does not fluctuate or a turbulence or a whirlpool is not generated in the flow, thereby stably forming an image having the high image quality.
In the present invention, the image formed on the image receiving medium includes graphical intelligence patterns such as alphanumeric characters, graphical display, line art, and other image information.